The government's motive for attempting to conceal evidence from Guantánamo Bay detainees became apparent last summer when a hoard of intelligence service policy and interrogation reports fell into the public domain.

The secret files, eventually disclosed through legal action brought by six British citizens and residents against MI5 and MI6, revealed the extent of the previous government's involvement in the illegal abduction and torture of its own citizens following the al-Qaida attacks of September 2001.

Among the most embarrassing documents were a series of interrogation reports from MI5 officers that betrayed their disregard for the suffering of a British resident they were questioning at a US airbase in Afghanistan. The documents also showed that the officers were content to see the mistreatment continue.

One of the excerpts was chapter 32 of MI6's general procedural manual, entitled Detainees and Detention Operations, which advises officers that among the "particular sensitivities" they need to consider before becoming directly involved in an operation to detain a terrorism suspect is the question of whether "detention, rather than killing, is the objective of the operation".

The files showed that the Foreign Office decided in January 2002 that the transfer of British citizens from Afghanistan to Guantánamo Bay was its preferred option, although Jack Straw asked for rendition to be delayed until MI5 had been able to interrogate those citizens.

The former detainees alleged that the intelligences services, the Home Office, the Foreign Office and the Attorney General's Office were complicit in their illegal detention and torture.

Before the claims were settled through payments, the government responded to disclosure requests by saying that it had identified up to 500,000 documents that could be relevant and it might take a decade to scrutinise them.

The cases of the six men are among those that will be considered by an inquiry headed by Sir Peter Gibson. Announcing the inquiry, Cameron told MPs last year: "This inquiry will be able to look at all the information relevant to its work, including secret information. It will have access to all relevant government papers – including those held by the intelligence services."

One of the six was Bisher al-Rawi, an Iraqi citizen, who had lived in Britain for 19 years when he was arrested in the Gambia in 2002 while setting up a peanut processing factory with his brother. He was questioned and handed over to the CIA after information was passed to the US and Gambian authorities by Britain's security services.

Rawi was taken to Bagram airbase in Afghanistan – where he alleges he was tortured – and then to the US base in Guantánamo Bay in Cuba. He was released and flown back to Britain in 2007.